Study On Geophysical Characterization Of Concentrated Gas Hydrate Reservoirs

Concentrated gas hydrate reservoirs are the leading targets for exploitation in the future given the existing techniques and engineering conditions. In the paper, two are chosen to represent the typical concentrated hydrate reservoirs, one from Krishan-Godavarin(KG) basin on the Indian margin with hydrate-filling in the fractures of fine-grain sediments, and the other from Green Canyon G955 in the Gulf of Mexico with hydrate-filling in the pores of sands. Multi-channel seismic(MCS), OBS, well logs, combined with regional geologic backgrounds and drilling materials, are used to characterize of the two types of gas hydrate reservoir, attempting to understand the discipline of gas hydrate concentration and its controlling factors.Site NGHP01-10 drilled the concentrated gas hydrate layer of around 135 depth in the fine-grain sediments in the KG basin of India. The well logs indicate gas hydrate fills in the fractures of high angles and the fractures are well arranged along certain direction, resulting in the anisotropy of reservoirs. The saturation of gas hydrate at Hole 10 D is estimated to be 24% averagely within the depth of 65-145 mbsf, and 30% at some depths using the combination of P-wave and S-wave velocity basing on the layered medium model. The bottom simulating reflector(BSR) is well recognized on the 2D seismic section of high resolution. BSRs show a characteristic of low amplitude and weak continuity. The absolute values of the reflection coefficients of BSR are mostly smaller than 0.1, except the BSRs near the faults. The comprehensive use of seismic impedance inversion, rock physics modelling and methane solubility analysis lead to a conclusion that the BSR amplitude is associated with the free gas beneath BSR, and the free gas is not uniformly distributed due to faults, and the free gas is thin.Hydrate in sands is drilled at the GC955 area in the Gulf of Mexico and the thickness of hydrate layer is about 30 meters at Hole H. The saturation of hydrate estimated from well log is about 30% averagely within the sand layer at Hole H. Hole Q drilled the boundary between the hydrate and free gas. The velocity model and depth image are obtained from a combination of traveltime inversion and depth migration using OBS and MCS line of high resolution passing through Hole H and Q. Furthermore, a fine velocity model was obtained from full waveform inversion using the OBS data. The strong reflectors on the seismic section were interpreted to be deepwater channel-levee deposits. The perturbation model between the FWI starting model and final model indicates the spatial distribution of gas hydrate and free gas. The base of gas hydrate stability zone was also implied from the perturbation model considering the interface of gas hydrate and free gas at wells. The attenuation was estimated from the sonic wavefroms at Hole H, showing the presence of hydrate increase the attenuation within the sands.